When it comes to dividing perennials, timing is crucial for successful plant propagation. While most plants thrive when divided in spring, some species benefit from being divided in late summer or fall. However, certain perennials, particularly semi-woody varieties, are not suited for division at all. Understanding the ideal timing for dividing perennials is essential to minimize stress on the plants and promote healthy growth.
The process of dividing a perennial involves digging up the plant, disrupting its roots, and replanting it in separate sections. This can be distressing for the plant, requiring weeks or even months to recover fully. Dividing plants before they flower consumes valuable energy that could otherwise be used for growth and blooming. Timing is crucial to avoid disrupting flowering cycles and weakening the plant, especially before extreme weather conditions like heatwaves or frost.
In general, spring- and summer-flowering perennials are best divided in the fall, while fall-flowering plants should be divided in the spring. Dividing perennials after flowering allows the plant to focus its energy on establishing new growth. Below are some popular perennials categorized by the best time for division, along with tips to help you determine the ideal approach for your garden.
For fall divisions, aim to complete the process at least 4 to 6 weeks before the first expected frost date in your region. In northern areas, this may mean dividing in late August or September, while southern gardeners can wait until the end of October. However, certain plants, such as woody perennial shrubs like roses and herbs, are better propagated through stem cuttings or layering rather than division. Plants with taproots are also unsuitable for dividing.
Plants that are not recommended for division include Rosemary, Lavender, Lupine, Russian sage, Butterfly weed, and Foxglove. When it comes to spring-flowering perennials like Peonies, Oriental Poppy, and Siberian Iris, it is best to wait until fall for division to ensure healthy growth and abundant flowering. Proper timing and careful handling are key to successful perennial division and garden maintenance.
Bearded irises (Iris × germanica) should be divided every 3-5 years as they tend to become overcrowded. Late summer is the ideal time for this task. Trim the leaves to a third of their height, dig up the clumps, and use a sharp soil knife to separate the rhizomes.
Ornamental onions, also known as alliums, grow from bulbs that multiply yearly. Divide them in the fall by digging up the clump and gently pulling it apart where it naturally separates. Make sure to mark their location in the summer with a stake or flag if they lose their foliage and are hard to locate in the autumn.
When the center of a group of globe flowers (Trollius chinensis) starts to thin out or if their flowering declines, it’s time to divide them. New transplants may take up to a year to recover from division and resume full flowering.
Divide spring-blooming Jack-in-the-pulpit plants (Arisaema triphyllum) after they have entered dormancy in the autumn. Protect your skin and wear gloves as the sap can be irritating. Gently break off any offsets by hand and replant them.
Marsh marigold (Caltha palustris) is a native plant that thrives in damp areas. Lift the entire plant in late summer or fall after flowering has ended. Carefully tease apart the clumps instead of using a tool to separate them.
Prairie smoke (Geum triflorum) is a stunning wildflower that forms large clumps. While it does not require division when growing in the prairie, in a garden setting, dividing it every few years can help maintain its vigor and flowering. Wait until the plant has finished flowering and setting seeds before dividing.
True lilies, plants from the Lilium genus, should be planted or divided in the fall. This category includes Asiatic, Oriental, and trumpet lilies, but excludes daylilies. Wait for the foliage to yellow and wither before dividing, and wear gloves as the sap can be irritating. Dig deeply, lift the clump, and look for natural divisions where the bulbs can be separated by hand.
